CLDN1 and metastatic melanoma: For example, claudin-1 was found to activate MMP-2 through the activation of the c-Abl-protein kinase Cδ (PKCδ) signaling pathway, while increased secretion of MMP-2 correlated with cytoplasmic expression of claudin-1 in metastatic melanoma cells in a PKC-dependent manner (78–80).